Quantitative determination of metals



Patented Jan. 29, 1952 Gerold Schwarzenbach, Zurich, Switzerland, as-

- signor to Chemische Fabrik Uetikon (Uetikon Chemical Company),Uetikon, Switzerland, a

firm of Switzerland No Drawing. Application April a, 1946, Serial No.660,408. In Switzerland May 2, 1945 iz'ciaims.

For the quantitative determination of numerous metals and, especially,of the alkaline-earth metals, at the present time only gravimetric andindirect titrimetric methods are available. Thus, for example, for thedetermination of the total hardness of a natural water by the standardmethod, a sample thereof is treated with an excess of a standardsolution of sodium carbonate and sodium hydroxide, in order toprecipitate the calcium as carbonate. This is effected either byevaporating themixture to dryness or letting it stand for a day. In theformer case it is taken up in distilled water. The whole is thenfiltered and the filtrate is titrated back with hydro chloric acid ofknown strength. The total hardness can be calculated from the volumes ofthe sample of water employed, the added solution of sodium carbonate andsodium hydroxide and the hydrochloric acid needed for the backtitration. This method is very complicated andtedious. In addition, theresult is obtained as a difference between two comparatively largefigures and therefore lacks accuracy. methods for determining the totalhardness are either similarly troublesome or are suitable only forapproximate determination of hardness as, for example, in the case ofBlachers method in which a soap solution is employed.

It has now been found that the total hardness of any water can bedetermined in a particularly simple and reliable manner by adjusting tothe same pH value a sample of the Water and a solution of an aminopolycarboxylic acid compound containing in relation to one basicnitrogen, atom more than one carboxymethyl, group attached directlythereto, said compound being capable of sequestering calcium in complexform, mixing with the sample of water an amount of thesolution. of aminopolycarboxylic acid compound at least sufficient to sequester in solublecomplex form the metal cations such as calcium and magnesium responsiblefor Water hardness, and then adding progressively a standard alkalinesolution until the pH value of the mixture is restored to the value towhich the water sample and the solution of amino polycarboxylic acid Icompound were initially adjusted.

The other known ltshould be here pointed out that the hydrogen which isassociated with the amino carboxylic compound is fairly tightly held andcontributes virtually no acidity to the aqueous solution in which theamino carboxylic acid compound is employed; When the solution is addedto the watersample, and the metal ions react with the amino carboxylicacid compound, thishydrogen is displaced and becomes an active hydrogenion and thereby produces a proportionate drop in the pH value of thefinal solution. Consequently, by titrating the solution with a standardalkaline solution such as dilute sodium hydroxide solution, until the pHvalue of the sample is restored to its original adjusted value whichexisted prior to the addition of the complex-forming aminopolycarboxylic acid compound, the volume of alkali solution required toneutralize the liberated acid represents the equivalent of hardnesspresent in the water sample.

. The pH adjustment of the original water sample and that of the aminopolycarboxylic acid compound may be readily effected eitherelectrometrically or more simply with the aid of one of the Well-knownacid-base colorimetric indicators, for example methyl orange, methylred, phenolphthalein, etc.

In addition to calcium, the other alkalineearth metals, and alsolithium, aluminium the rare-earth metals, zinc, cadmium, mercury, lead,copper, iron, cobalt, nickel and manganese can be quantitativelydetermined by the same method. Especially suitable as complex-formingcompounds are the aminopolycarboxylic acids of the general formula CHrC0 0H CHz-C 0 0H wherein R stands for a member selected. from the groupconsisting of hydrogen and the radicals Iminodiacetic acid,nitrilotriacetic acid, anthraacid-diacetic acid and uramildiacetic acidhave, recently been prepared by me (Helvetica chimica acta 28, 1141(1945); 29, 368 (1946)).

The following are suitable as complex-forming compounds: the salts ofiminodiacetic acid,

and of its derivatives, for example, nitrilotriaceimportance of the newmethod lies in the fact that it enables a considerable number of metalsto be titrated with the same standard alkaline solution as. that usedfor the usual allialimetric titratiohsl Nospecial chemicals or apparatusare necessary for these determinations in addi tion to thecomplex-forming substances mentioned. Also, some of the complex-formingcompounds are easily obtainable in an excellently crystalline form andina condition. of absolute purity and are therefore pre-emi'nentlysuitable as substances for titrimetric standards with the aid ofwhichthe alkaline and all other stand: ard solutions arerea'dily. adjustable.The accuracyof the metal determinations is increased by the employmentof such adjusted standard alkaline solutions. Asia modification of themethod outlined above, instead of adding, a standard alkali solution andtitrating. with the aid of one of the acid-base. colorimetric indicatorsmentioned, any one. of a. number of colorimetric indicators particularlysuitable for determination of pH may be addedto a. solution, forexample, containing water sample and complex-forming compound. Thechoice of indicator will be de pendent upon the pH range in whichtheanalysis complex-former, the next increment of alkali complex-formersolution will cause a readily detectable increase in pH value which canbe rendered visible by means of a suitable indicator. Thus, cobalt,nickel, zinc, copper and lead can be titrated very accurately with thetripotassium salt of nitrilotriacetic acid, whilst,'for cadmium andmanganese, it is better to employ the tetrabe effectedpetentiometrically by means of an indicator electrode.

It has furthermore been found that in the titration of inetals by meansof a standard solution of the complex-forming compound, there can alsobe; employed an indicator which responds directly by color change to theions of the metal to be determined, some of these metal ions combiningdirectly with the indicator itself. Thus, for example, iron and cobaltcan be ti trated with the complex-former using thiooya nate as theindicator and taking as the end po'int the vanishing red or bluecoloration depending upon the metal ion present. For determining zinc,for example, the diphenylthiocarbazone known by the name dithizone'm'aybe employed as an indicator. Especially impressive is the ti tration ofcalcium with murexide as the indi cator. Murexide (ammonium purpurat'e)gives a blue color in alkaline solution and a red color in the presenceof calcium. Accordingly, if' asolution of a calcium salt and murexide,said s0= dicator responds immediately by changing color.

to thewater sample is a measure of the total hardness present in thewater sample. The above subject matter is claimed in my divisional,co-pending application, Serial. No. 195,473, filed November 13, 1950.

It has further been found that the concentra tion of the v'ario'usmetals above enumerated can also be determined by titrating a solutioncontaining the metal ion with a standard solution of analkaline-reacting salt of one of the complex-forming compounds. Here noacid is liberated by the reaction between the metal ion and thecomplex-former as in the preceding exam:

ples. Instead, so long as all the metal ionshave not reacted'with' thepolycarbox-ylic acid saltto forma complex, the pH value of the solutionwill. remain substantially unchanged. However,

' when. a. balance or equilibrium is, reached and" all or the ine'talions liave ben. iihited withfthe, j

lution having been rendered alkaline, is titrated with a standardsolution of the complex-forming compound, the red color or the indicatorplex-for'ming compound. Such an indicator functions on'an entirelydifferent basis than indicators. The latter respond solely to pH changeswhile the 'former: undergo color change in solution in the presence ofthe metal or metals I to be detected". The change-in color occurswhenamazing-tor calcium by this Method since the" complex-formingcompound tiesupall of the 1111- combin'ed calcium in solutionand-finally extracts the calcium which had previously combined withthemurexide indicator. When this final extraction of the previouslycombined calcium occurs,- the indicator changescolor. Since the murexidedoes not-respond to magnesium ions, this method is suitable forthedetermination of calcium in the presence'of magnesium. V

For these determinations. of: metals, there is advantageouslyselectedfrom the complex-forming compounds. mentioned a. compound havingan equilibrium constant of formation of the cominlexlwith the. metal:to. be determined which, is. as large as; possible and' amounts. to atleast; 10(1'.

Examples 1;. A natural, water, the calcium content oil which, accordingto the known standard method; hereinbeiore mentioned; amounted to 24.4.French. degrees, was to be analysed- A sample. measuring 200 C05} was,after the, addition. or methyl red, brought to a. value; of r 5 withthe, addition of 0.021s. hydrochloric acid. "incidem, tally, thfnuinbrdice-required for thisv a by 2', ves the temporary .harenes new 9estimat on. I

contained 20 gms. of uramildiacetic acid per litre, neutralised to a pHvalue of 5, the solution; which became acid, was titrated with 0.02Ncaustic soda solution until the color of the solution was the same shadeof color asthat of the original water sample after addition of methylred and the 0.02N hydrochloric acid. 24.3 cc." were, required,corresponding to a total hardness of 24.3 French degrees. Instead ofmethyl red, bromocresol purple can be employed with the same success. 9

' If ethylenediamine-tetracetic acid is employed instead ofuramildiacetic acid, twice the amount to 0.02N caustic soda solution isrequired, so that the number of cc. of standard solution run in is to bedivided by -2 to obtain the total hardness in French degrees. Thus, 200cc. of the same water, after adjustment to a pH value of 5 andsubsequent-addition or a solution of ethylenediamine-tetracetic acid,the pH value of which also amounted to 5, required 49.0 cc. of 0.02Ncaustic soda solution, corresponding to a hardness of 24.5 Frenchdegrees.

2. A sample of 200 cc. of the natural water employed in Example 1 wastreated with cc. of normal caustic soda solution, a few drops of anaqueous solution of murexide were added and the wholewas titrated with astrongly alkaline solution of the complex-forming compound until a colorchange took place from redto blue-violet. The standard solution employedwas one containing 5.84 gms. of ethylenediaminetetracetic acid. Only thecalcium but not the magnesium, is determined by the change of color ofthe murexide, so that only the calcium hardness is obtained, becausethis standard solution causes the calcium ion to disappear before themagnesium ion.

3. For the purpose of determining the lead in a solution, produced bydissolving 6.96 gms. of the purest lead chloride per litre, cc. weretreated with 1 gm. of sodium chloride and, after the addition of methylred, were brought to a pH value of 5 with caustic soda solution. 40cc..of a m./40 solution of ethylenediamine-tetracetic acid, which alsohad a pH value of 5, were then added and the solution was. titrated backwith 0.05N caustic soda solution to a pH value of 5, for which purpose19.9 cc. of the standard solution were necessary. The calculatedquantity of lead was 0.103 gm., whilst 0.1025 gm. was found.

4. For the purpose of determining zinc, there were employed 20 cc. of a0.1 m. solution of zinc chloride the content of which was determinedgravimetrically. After adding bromothymol blue, the whole was titratedwith a standard solution. of 0.1 m. of trisodium nitrilotriacetate untila very sharp change of colour took place. For this purpose, exactly 20cc., i. e. exactly the theoretical quantity.--wererequired. The standardsolution was prepared by dissolving 19.1 gms. ofthe purestnitrilotriacetic acid, dried in vacuo, in'300"cc.-of'-normal causticsoda solution and making up to one litre.

5. For the purposeof determining copper, 20 cc. .of a 0.1m. solution ofcupric chloride andthe same standard solution as in Example 4 were;employed, but methyl red was employed as} indicator." The color changewas from lilac to pure. blue. In order to reach this color change, 19.8cc. of the standard solution were necessary, whilst theoretically 20 cc.should have been used.

The following are the formulae or the acids hereinbefore mentioned fromwhich the complex-forming salts are produced:

20 C O OH--C 5 NIL-06 CHr-C 0 OH CHs-C 0 OH Imidodiacetic acid CHr-C 00H 0 Nitrilotriacetic acid COOH /CHr-G 005 N Anthranilie-acid diaceticacid COOK-43g, QH -COQH 4 N-CH|OHrN i CHr-G 0 OHEthylenediamine-tetracetic acid NH-C 0 CHr-C 0 OBI o6 CH- CHr-C O OBIUramildiacetic acid 0 0 0H CHI-C 0 on CHN CHr-C 0 0H Aminomalonic-acid(acetic acid What I claim is: 1. A method of volumetrically determining1 metal cations in solution which comprises titrating a measured amountof the solution with a solution containing a known concentration of anamino polycarboxylic acid compound, said compound having in relation toone basic nitrogen atom more than one carbcxymethyl group directlyattached thereto, until an end-point detectable by suitable indicatingmeans is reached, said end-point being the point where all of theaforesaid metal cations are united with the anions of the aminopolycarboxylic acid compound to form 45 a complex ion thereof, theamount of amino polycarboxylic acid compound being a direct measure I ofthe aforesaid metal cations present in solution. 2. A method ofdetermining metal cations in solution which comprises titrating thesolution with a solution containing a known concentration. of a compoundof an amino polycarboxylic acid compound of the general formula CHzCOOHwhere R' is a member selected from the group consisting of hydrogen andthe radicals until an end-point detectable by suitable indicata metalcations present in solution. a

3. A method of volumetric-ally determining assassinpound being a directmeasure" of the aforesaid metal cations in solution which comprisestitrating a measured amount of the-solution with a.

solution containing a known concentration of an alkaline-reacting saltofan amino polycarboxylic acid compound said compound having in relationto one basic nitrogen atom more than one carboxymethyl group directlyattached thereto, until an end-point is reached, said end-point beingthe point'where-all ofthe aforesaid metal cations are united with theanions of the amino polycarboxylic acid compound to form a complex ionthereof, and being the point where there occurs an increase in pH valuewhich is'detectable by suitable indicating means;- the amount ofalkaline-reacting salt of amino polycarboxylic acid compound beingadirect measure of the aforesaid metal cations present in solution.

4. A method according to claiin 3 in which the amino polycarboiiylicacid has the general formula emotion cfiiceoa where R is a memberselected from the group eonsistingf or hydr gen and the radicals r ICHZC OOH 1 and 5. A method according to claim 8 in which-the" ami-nopolycarboxylic acid compound is a salt" of metal cations in solutionwhich comprises making the solution suitably alkaline, titrating ameasured amount of the solution with a solution containing a knownconcentrationiof an amine polycarboxylic acid compound; said compoundhaving in relation to one basic nitrogen atom more thanone-'carboxy'methyl group directly attached thereto, said titratioribeing carried out iii-the'presence 'of a suitable colorimetric indicatorwhich is responsive to the presence and absence of the aforesaid metalcations in solution, until a visible c ange in the color of theindicator mugs. said change occurring when all the afar are metalcations are coiiibined with the" anions of the amino polycarboxvlicacidcompound, there being substantially no change in the" pH valueersolution, the volume of amino polycarbo'iiyli a d coinnoiind beinga'direct measure of tiieambcnt of the aforesaid metal cations present intheoriiriaisciuubn. v I V 9'. A method according to claiin 8'1 aminopolycarboxylic acid has the g mind consisting of hydrogen and theradicals and salts thereof.

10. A method according to claim 8'in which the amino 'p'ol'ycarboxylicacid compound-is a salt of ethylene diamine tetra acetic acid.

11. A method according to claim 8 in which the amindpolycarboxylic acidcompound is asalt ofuramildiacetic acid.

12 A method according to claini 8 in which the amino. polycarboxylicacid compound is asalt of nitrilo triacetic acid.

a 1 GEROLD SCHWARZE ACE-I.

. REFERENCES CITED The renew ng references are of fecord 1h me of thispatenti UNITED STATES PATENTS OTHER REFERENCES" 7 'i a'yloi' and Baker oganic Chemistry of itrogen," page 122 (1942); Oxford University Prssy.Chein; and Met. Eiig; fMeasureinent and-Control of pH, pages 553-560,August- 1940.

1. A METHOD OF VOLUMETRICALLY DETERMINING METAL CATIONS IN SOLUTIONWHICH COMPRISES TITRATING A MEASURED AMOUNT OF THE SOLUTION WITH ASOLUTION CONTAINING A KNOWN CONCENTRATION OF AN AMINO POLYCARBOXCYLICACID COMPOUND, SAID COMPOUND HAVING IN RELATION TO ONE BASIC NITROGENATOM MORE THAN ONE CARBOXYMETHYL GROUP DIRECTLY ATTACHED THERETO, UNTILAND END-POINT DETECTABLE BY SUITABLE INDICATING MEANS IS REACHED, SAIDEND-POINT BEING THE POINT WHERE ALL OF THE AFORESAID METAL CATIONS AREUNITED WITH THE ANIONS OF THE AMINO POLYCARBOXYLIC ACID COMPOUNDS TOFORM A COMPLEX ION THEREOF, THE AMOUNT OF AMINO POLYCARBOXYLIC ACIDCOMPOUND BEING A DIRECT MEASURE OF THE AFORESAID METAL CATIONS PRESENTIN SOLUTION